This invention relates to belt drives in which toothed power transmission belts operate in conjunction with toothed gears or pulleys and to belts and pulleys adapted for use in such drives. More particularly this invention relates to the specific dimensional relationship between the height of the belt driving teeth and the height of the pulley teeth or depth of the pulley grooves between the pulley teeth. The invention is most particularly related to synchronous or positive drive systems in which a positive drive power transmission belt operates in conjunction with a pair of toothed pulleys.
Power transmission belts used with toothed pulleys are well-known in the art. These belts have a plurality of alternating teeth and grooves extending generally transversely of the belt which mesh with alternating teeth and grooves of the toothed pulley or sprocket in order to perform their driving function. The most widely used of these toothed belts are the so-called synchronous or positive drive belts which are manufactured from flexible resilient material such as natural or synthetic rubber. These belts are engineered and manufactured with pitch, tooth depth, width and other measurements accurate to a precise degree with extremely close tolerances being maintained. In addition, a high strength tensile stress resisting member of essentially inextensible material is provided substantially on the dedendum line of the teeth to prevent undue stretchability of the belt. This belt construction allows the flexible, resilient belt teeth to mesh without substantial change of pitch with teeth of the toothed pulleys with the belt thereby functioning as a synchronizing belt. The operation and advantages of synchronous drive belts are fully described in Case U.S. Pat. No. 2,507,852.
Conventionally in the prior art the dimensional relationship of the belt teeth and grooves and the pulley teeth and grooves has been such that in the longitudinal extent of the belt between the pulleys the height of the belt teeth is equal to or less than the depth of the pulley grooves. Consequently, as the belt travels around the pulleys either a line to line contact is established between the extreme outwardly facing portions of the belt teeth which confront the pulley and the portions of the pulley disposed between the pulley teeth which define the bottom of the pulley grooves or there is a clearance between these stated portions of the belt teeth and pulley grooves. An exception to the dimensional relationship stated above is disclosed in Elster U.S. Pat. No. 3,313,166 in which the height of the belt teeth is disclosed as being greater than the height of the pulley teeth. This results in a clearance between the extreme radially outwardly facing portions of the pulley tooth and the portions of the belt disposed between the belt teeth which define the bottoms of the belt grooves with the pulley teeth thereby engaging the belt teeth at the tooth flanks only.
Typically the belt driving teeth of the positive drive belt have been of a rectilinear cross-sectional configuration and most commonly have been of a trapezoidal configuration when viewed in longitudinal cross-section. The spacing or width of the grooves between the belt teeth originally was designed so as to be greater than the width of the root or base of the belt tooth. However, problems were encountered with belt failure due to shearing of the belt teeth resulting from the concentration of stresses in the belt. Consequently, many efforts were made to modify the existing belt tooth profile in order to solve the tooth shear problem. For example, in order to achieve a more satisfactory distribution of the load or stress on the belt, the number of belt teeth for a given length of belt has been increased. In addition, a larger size of individual belt tooth has been manufactured to provide a greater resistance to tooth shear. These approaches naturally have decreased the space between belt teeth so that the width of the root or base of the belt tooth is equal to or greater than the space between the teeth.
More recently various changes in the contour or configuration of the belt tooth have been made together with the approaches discussed above. For example, Miller U.S. Pat. No. 3,756,091 discloses a positive drive system in which the belt includes relatively closely spaced together driving teeth of a special curvilinear cross-sectional configuration which is operated in conjunction with pulley teeth having mating substantially conjugate curvilinear cross-sections.
Problems have been encountered in conventional positive drive systems with the existing dimensional relationships between the belt and the pulley teeth or grooves. Moreover, the belt modifications as discussed previously which were implemented to counter the belt tooth shear problem have created other problems due to the relative spacing of the belt teeth. For example, the tensile member of the positive drive belts in the heretofore existing systems have not been adequately supported by the teeth of the belt and therefore have been subjected to excessive stress in the area between teeth during the operation of the drive. This problem has been particularly acute in drives wherein the base or root of the belt tooth as measured substantially on the dedendum line of the tooth is greater than the space between belt teeth as measured substantially on this same line. In this instance the pulley teeth which contact the spaces between the belt teeth have a relatively small apex and the condition resulting may be likened to the belt passing over knife blades. Additionally, excessive vibration and undesirable noise also have been encountered.
It has been found that the disadvantages of the prior art can be overcome by the present invention which will be hereinafter described.